The present invention relates to a method and means for accurately aligning the axes of two or more rotary parts, particularly to a method and means for accurately aligning the axes of shafts in pump assemblies for use in nuclear reactor plants.
It is known to drive the impeller of a centrifugal pump in a nuclear reactor plant by a motor through the medium of face gears which transmit torque from the motor shaft to the pump shaft. In such plants, the hydrodynamic or hydrostatic shaft seals which prevent escape or control the leakage of contaminated fluid from the pump body must be readily accessible for inspection, adjustment and/or replacement. The pump shaft rotates in one or more bearings which are lubricated by the circulating fluid, and the motor shaft rotates in one or more bearings which are lubricated by oil. The hydrodynamic or hydrostatic shaft seal is disposed between the two sets of bearings. If the motor shaft is integral with the pump shaft, the entire motor must be dismantled, with attendant substantial losses in time, whenever the attendants desire to gain access to the shaft seal.
In accordance with certain prior proposals one of which is disclosed in the commonly owned copending application Ser. No. 432,343 filed Jan. 10, 1974 by Peter Stech, the motor shaft transmits torque to the pump shaft through the medium of an intermediate shaft which is removable to afford convenient access to the shaft seal with greatly reduced losses in time. The intermediate shaft actually constitutes an extension of the pump shaft and is separably coupled thereto directly above the housing for the shaft seal. Once the intermediate shaft is removed, the attendants are in a position to reach not only the shaft seal but also the bearing or bearings for the pump shaft. An advantage of such pump assemblies is that the motor and the bearing or bearings for the motor shaft need not be dismantled or displaced at all.
However, the utilization of the just discussed intermediate shaft presents certain other problems, especially upon repeated removal and reinsertion of the intermediate shaft. Thus, it is difficult to move the axis of the reinserted intermediate shaft into exact alignment with the other two shafts so that the pump assembly exhibits a pronounced tendency to vibrate with attendant wear upon the bearings for the pump shaft and/or motor shaft. Moreover, pronounced vibrations reduce the effectiveness of the shaft seal so that the quantity of leak fluid increases; this is particularly undesirable when the leak fluid contains a radioactive substance. Attempts to reduce vibrations of a composite torque-transmitting connection between the motor and the impeller of the pump include the provision of a heat-shrunk connection (normally assisted by a key or tongue-and-groove joint) between separable portions of the motor shaft, between separable portions of the pump shaft or between the pump and motor shafts on the one hand and the intermediate shaft on the other hand. It was also proposed to pump oil or another liquid at a very high pressure between the neighboring flanges of cooperating shafts. Such techniques are not entirely satisfactory because they are not reproducible with a sufficient degree of accuracy. This is due to the fact that a connection which is obtained by heat-shrinking and/or by filling the joint with a highly pressurized liquid transmits torque by friction. When the joint is to be dismantled, the frictionally engaged parts undergo a pronounced deformation as a result of cooling or in response to a reduction of liquid pressure. Even minor deviations from the original degree of deformation (upon reestablishment of the joint by heat-shrinking or with a highly pressurized liquid) cause a pronounced wobbling and attendant vibrations with the aforediscussed adverse effects upon the shaft seal and bearings. The making of joints by heat-shrinking exhibits the additional drawback that the joint must be cooled to room temperature before the pump assembly can be put to renewed use, a procedure which normally takes up a period of 6-8 hours. Thus, each inspection of the shaft seal necessitates a very long interruption of operation of the pump assembly.